Elements of chemistry, including the recent discoveries and doctrines of the science / by Edward Turner.

  • Turner, Edward, 1798-1837.
Date:
1833
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry, including the recent discoveries and doctrines of the science / by Edward Turner. Source: Wellcome Collection.

    68/1026 (page 40)
    - The mercurial thermometer may be made to indicate tem- peratures which exceed 212°, or fall below zero, by continu- ing the degrees above and below those points. But as mer- cury freezes at 39 degrees below zero, it cannot indicate temperatures below that point; and indeed the only liquid which can be used for such purposes is alcohol. Our means of estimating high degrees of heat are as yet very unsatisfac- tory. Mercury is preferable to any other liquid; but even its indications cannot be altogether relied on. For, in the first place, its expansion for equal increments of caloric is ereater at high than at low temperatures; and, secondly, glass expands at temperatures beyond 212° F. in a more rapid ratio than mercury, and consequently, from the pro- portionally greater capacity of the bulb, the apparent expan- sion of the metal is considerably less than its actual dilatation. Thus Dulong and Petit observed, that when the air thermo- meter is at 572° F., the common mercurial thermometer stands at 586°; but when corrected for the error caused by the glass, it indicates a temperature of 597.5° F. No liquid can be employed for temperatures which exceed 662° F., since all of them are then either dissipated in vapour, or de- composed. M. Bellain has observed that mercurial thermometers slowly change their point. of zero, which uniformly becomes higher than at the time of graduation. This phenomenon appears owing to a diminished capacity of the bulb due to the atmosphere continually pressing on its exterior, while a vacuum exists in the interior of the tube; for it has not been noticed either in mercurial thermometers which are un- sealed, or in thermometers made with alcohol. The princi- pal contraction ensues soon after the tube is sealed, and hence some months should be permitted to elapse between the sealing and graduation of a thermometer. (An de Ch. et Ph. xxi. 330.) The instruments for measuring intense degrees of heat are called pyrometers, and must be formed either of solid or gaseous substances. The former alone have been hitherto employed, though the latter, from the greater uniformity with which they expand, are better calculated for the pur- pose. The action of most pyrometers depends on the elon- gation of a metallic bar by heat; and the difficulty in their